1. Field of the Invention
The present invention relates to a semiconductor laser device in which the saturation of a light emission efficiency in a high power operation is suppressed, and an optical pickup apparatus using the same.
2. Description of Related Art
A semiconductor laser device (in the following, also referred to as a semiconductor laser) is in wide use in various fields. For example, since an AlGaInP semiconductor laser emits red laser light in a wavelength band of 650 nm, it is used widely as a light source in the field of optical disk systems such as DVDs. As an exemplary structure of the semiconductor laser, a double hetero structure including an active layer and two cladding layers sandwiching the active layer, in which one of the cladding layers forms a mesa-shaped ridge is known (see JP 2001-196694 A, for example).
FIG. 10 shows an example of an AlGaInP semiconductor laser having such a structure. The composition ratio of each layer described below will be omitted. In the semiconductor laser shown in FIG. 10, an n-type GaAs buffer layer 102, an n-type GaInP buffer layer 103 and an n-type (AlGa)InP cladding layer 104 are stacked in this order on an n-type GaAs substrate 101 having a plane tilted by 15° in a [011] direction from a (100) plane as a principal plane. Furthermore, a strained-quantum well active layer 105, a p-type (AlGa)InP first cladding layer 106, a p-type (or undoped) GaInP etching stop layer 107, a p-type (AlGa)InP second cladding layer 108, a p-type GaInP intermediate layer 109 and a p-type GaAs cap layer 110 are stacked on the n-type (AlGa)InP cladding layer 104. Here, the p-type (AlGa)InP second cladding layer 108, the p-type GaInP intermediate layer 109 and the p-type GaAs cap layer 110 are formed as a ridge having a regular mesa shape on the p-type GaInP etching stop layer 107. Furthermore, an n-type GaAs current blocking layer 111 is formed on the p-type GaInP etching stop layer 107 and on the side surfaces of the ridge. A p-type GaAs contact layer 112 is stacked on the n-type GaAs current blocking layer 111 and the p-type GaAs cap layer 110 located in the upper portion of the ridge. The strained-quantum well active layer 105 includes an (AlGa)InP layer and a GaInP layer.
In the semiconductor laser shown in FIG. 10, an electric current injected from the p-type GaAs contact layer 112 is confined to the ridge portion alone by the n-type GaAs current blocking layer 111 and concentrates into the strained-quantum well active layer 105 near a bottom of the ridge. Thus, in spite of the amount of injected current being as small as several tens of mA, a population inversion state of carriers required for laser oscillation is achieved. At this time, the recombination of carriers generates light. Then, in a direction perpendicular to the strained-quantum well active layer 105, the light is confined by both of the n-type (AlGa)InP cladding layer 104 and the p-type (AlGa)InP first cladding layer 106, and in a direction parallel with the strained-quantum well active layer 105, the GaAs current blocking layer 111 absorbs the generated light, thereby performing light confinement. Consequently, when the gain obtained by the injected current exceeds the loss in a waveguide in the strained-quantum well active layer 105, laser oscillation occurs.
In such a semiconductor laser, it is desired that highest possible light output should be achieved for rewriting an optical disk at a high speed. For example, in order to achieve at least a quad-speed rewriting for a DVD optical disk, a light output as high as 100 mW or more is required. For obtaining such a high output power, it is necessary to prevent a COD (catastrophic optical damage), in which an end face of a semiconductor laser is melted down and broken by its own light output at the time of high power operation. The COD is prevented effectively by reducing a light density inside the end face of a resonator of the laser so as to suppress heat generation. For this purpose, coating a front end face of the semiconductor laser for extracting laser light with a dielectric such as SiO2, Al2O3 or amorphous Si so as to reduce its reflectivity is effective.
In general, the reflectivity of the end face of the resonator of AlGaInP and AlGaAs semiconductor lasers is about 30% when the end face is not coated. In this case, about 30% of the laser light is reflected by the resonator end face and fed back to the inside of the resonator, while about 70% of the laser light is extracted from the front end face. In contrast, when the front end face is coated with a dielectric film to have a reflectivity of 10%, 10% of the laser light is reflected by the resonator end face and fed back to the inside of the resonator, while 90% thereof is extracted from the front end face. In other words, in the case where the same light output is extracted from the front end face, when the reflectivity of the front end face is reduced to ⅓, it also is possible to reduce the light density of the resonator end face to ⅓. Accordingly, reducing the reflectivity of the front end face increases a COD level and thus is effective in obtaining a high-power laser. Furthermore, if the reflectivity of a rear end face, which is an opposite side of the resonator face from which the laser light is extracted, is set to be high, it is possible to enhance a light extraction efficiency from the front end face of the semiconductor laser. Therefore, in a general high-power semiconductor laser, an end face coating condition of reducing the reflectivity of the front end face and increasing that of the rear end face is used widely.
As described above, in order to obtain a high-power laser, the reduction in the reflectivity of the front end face and the increase in that of the rear end face are effective in improving the COD level and the light extraction efficiency. However, when the reflectivity of the front end face is reduced excessively, the laser light that is fed back inside the resonator decreases, raising an oscillation threshold current. Also, in the case of applying the semiconductor laser to an optical disk, when the reflectivity of the front end face is low, it becomes likely that reflected light returning from the optical disk may generate noise (noise induced by returning light). Accordingly, in a high-power laser, the front end face usually is coated such that its reflectivity is about 5% to 10% so as both to achieve a high light extraction efficiency and reduce the noise induced by returning light. Further, the rear end face is coated so as to achieve a highest possible reflectivity, namely, about 95% to 100% in general.
As described above, the front end face and the rear end face of the high-power laser have considerably different reflectivities. In such cases, the distribution of the intensity of light propagating through the active layer in a resonator direction is not front-rear symmetrical with respect to the resonator but front-rear asymmetrical as shown in FIG. 2 where the front end face side has a higher intensity in light intensity distribution. FIG. 2 shows a light distribution in the resonator direction of a device whose resonator length is 1100 μm, front end face reflectivity is 7% and rear end face reflectivity is 95%, as an example.
In this case, since a more intensive stimulated emission occurs on the front end face side having a higher light intensity than on the rear end face side, more electron-hole pairs have to be injected into the active layer on the front end face side than that on the rear end face side. Especially in a high power operation, the front end face side becomes short of the electron-hole pairs in the active layer, which may cause a saturation of a light emission efficiency. This saturation of the light emission efficiency may degrade temperature characteristics of a high-power laser of 200 to 300 mW or higher, thus posing a serious problem.